Human eyes contain cones having three different spectral sensitivities, which is the basis of human color vision. Typical human cone spectral sensitivities are shown in FIG. 1, and are labeled ρ, γ and β. It can be seen that the peak sensitivities for each of the different cone types occurs at different wavelengths of light. The β cones are most sensitive to “blue” light of about 450 nm, whereas the γ and ρ cones have peak sensitivities at about 550 nm and 580 nm, respectively, covering the “green” and “red” portions of the spectrum. Objects of different colors are distinguished by having different spectral reflectances, and therefore will stimulate different relative responses in each of the different cone types. A key principal for understanding color vision is that any two objects that produce the same cone responses will be seen to have the same color. Color is typically measured by calculating the well-known CIE XYZ tristimulus values:
                              X          =                                    ∫              400              700                        ⁢                                          I                ⁡                                  (                  λ                  )                                            ⁢                              R                ⁡                                  (                  λ                  )                                            ⁢                                                x                  _                                ⁡                                  (                  λ                  )                                            ⁢                                                          ⁢                              ⅆ                λ                                                    ⁢                                  ⁢                  Y          =                                    ∫              400              700                        ⁢                                          I                ⁡                                  (                  λ                  )                                            ⁢                              R                ⁡                                  (                  λ                  )                                            ⁢                                                y                  _                                ⁡                                  (                  λ                  )                                            ⁢                                                          ⁢                              ⅆ                λ                                                    ⁢                                  ⁢                  Z          =                                    ∫              400              700                        ⁢                                          I                ⁡                                  (                  λ                  )                                            ⁢                              R                ⁡                                  (                  λ                  )                                            ⁢                                                z                  _                                ⁡                                  (                  λ                  )                                            ⁢                                                          ⁢                              ⅆ                λ                                                                        (        1        )            where λ is the wavelength, I(λ) is the spectral power of the light source, R(λ) is the spectral reflectance of the object, and x(λ), y(λ) and z(λ) are the color matching functions, which are convenient linear combinations of the cone spectral sensitivity functions. Any two objects having the same XYZ values will appear to have the same color to a human observer.
Often it is desirable to be able to scan a color image to form a digital image that can be viewed, manipulated, printed or stored in a digital computer. Because of the tri-chromatic nature of human color vision, a digital color image scanner must fundamentally have three different types of color sensors in order to be able to infer the color of the original image. The three different types of color sensors are generally chosen to be sensitive to the red, green and blue regions of the visible spectrum.
A set of typical spectral sensitivities for a digital color image scanner are shown in FIG. 2. It can be seen that these spectral sensitivities differ substantially from those of the human vision system shown in FIG. 1. This fact gives rise to a phenomenon known as scanner metamerism errors. This occurs when two objects match when viewed by a human observer, but which do not match when they are scanned with the scanner. Consider the case when two images formed on different input media having colorants with different spectral characteristics, are scanned on a digital color image scanner. If both images contain an image region of neutral gray, which appear to be identical to the human eye, it would be desirable that the scanner would produce identical scanner code values for these image regions. However, it is commonly found that the two image regions can produce substantially different scanner code values. This occurs because the two neutral gray regions may have very different spectral properties as is illustrated in FIG. 3. Such pairs of colors that are different spectrally, but which appear to be the same visually are commonly called “metamaric colors.”
Using well-known color management techniques, it is possible to relate the scanner RGB code values to the corresponding color as perceived by the human vision system. The perceived colors are commonly represented in color spaces such as the well-known CIE XYZ and CIELAB color spaces. However, since the way the scanner “sees” color is dependent on the spectral characteristics of the input media, the relationship between the scanner RGB code values and the perceived color will be different for different input media. Therefore if a color transform is created to map scanner RGB code values to corresponding CIELAB color values, this color transform will only be accurate for one particular input media. More specifically, different color transforms will generally be necessary for input images that are formed using different sets of colorants.
A flow diagram showing a typical color management process that can be used to transform scanner RGB images to a desired output color space associated with a particular output device is shown in FIG. 4. Scanner RGB color values 40 produced by a digital color image scanner are first transformed using a scanner input color profile 41, which relates the scanner RGB color values 40 to corresponding device-independent color values 42. The device-independent color values 42 will generally be in a color space such as CIELAB or CIE XYZ. The device-independent color values 42 are then transformed using an output device color profile 43 to produce output device color values 44 appropriate for displaying/printing the scanned image on the particular output device.
The scanner input color profile 41 is usually determined for one particular input media, which will be designated as the reference input media. As described earlier, if the scanner is used to scan input media using different colorants than the reference input media, the relationship between the scanner RGB color values 40 and the device independent color values 42 will generally be different. As a result, the scanner input color profile 41 will not produce accurate results. In some cases the inaccuracies can be very dramatic and quite objectionable.
For most applications, it is desirable to be able to scan different types of input media on a given digital color image scanner. Therefore, it is necessary to deal with any scanner metamerism errors that may be associated with a given scanner/input media combination. The simplest approach is to design the scanning system for a single reference input medium, and then to live with any errors that are introduced for other input media. However, in many cases this can be quite unsatisfying if the scanner metamerism errors are substantial.
An alternative solution is to determine different scanner input color profiles 41 for each different input medium. While this approach can provide very accurate color reproduction, it is necessary for the user to select the appropriate color transform for the input medium that is being scanned. This can result in a very confusing and cumbersome user interface.
This problem can be particularly awkward when designing an all-in-one printer having both a scanner and a printer in a single mechanism. For many applications, the most common type of image that will typically be scanned on such systems will be legacy photographs printed on a conventional photographic silver halide print medium. As a result, this is the medium for which the system designer would generally want to optimize the scanner color correction transform. However, another common input that would be scanned would be images printed on the associated printer in the all-in-one system. And frequently the colorants used on that printer will be significantly different than those of the conventional photographic silver halide print medium. If the spectral sensitivities of the scanner differ significantly from human color matching functions, the result will be unsatisfactory color reproduction when these images are scanned. This can provide a high level of dissatisfaction for the user since they will generally expect that the system should be able to produce a high quality copy of an image that was printed on that system.